The invention relates to statistical multiplexing in a telecommunications network, as well as a statistical multiplexer, and statistical demultiplexer.
FIG. 1 in the accompanying drawings shows a simplified block diagram of GSM mobile communication system (GSM. Global System for Mobile Telecommunications). The network subsystem (NSS) comprises a mobile services switching centre MSC which communicates with other mobile services switching centres, and either directly or via the system interface of a gateway mobile services switching centre (GMSC), the mobile communication system is connected to other networks, such as the public switched telephone network (PSTN), an integrated services digital network (ISDN), other mobile communication networks such as the public land mobile network (PLMN) and packet-swztched public data networks (PSPDN) and circuitswitched public data networks (CSPDN). The mobile services swishing centre comprises network interworking functions (IWF) by means of which the GSM network can be adapted to other networks. The network subsystem NSS is connected via the A interface to the base station subsystem (BSS) which comprises base station controllers BSC, each controlling base stations BTS that are connected to them. The interface between the BSC and base stations BTS connected thereto is the A bis interface.Base stations BTS for their part communicate on the radio path with mobile stations MS over the radio interface. The operation of the entire system is monitored by an Operation and Maintenance Centre, OMC.
The transcoder/rate adaptor unit (TRAU) is a part of the base station subsystem BSS and may be located at the base station controller BSC, as shown in FIG. 1, or alternatively at the mobile services switching centre MSC. The transcoders convert speech from a digital format to another, for example 64 kbit/s PCM received over the A interface from the MSC, into data to be transmitted to the base station, and vice versa. One 64 kbit/s PCM channel carries four speech/data connections, which means that the rate of one speech/data channel on this link is 16 kbit/s.
The mobile station MS transmits user data over the radio interface on the radio channel at 13 kbit/s or 5.6 kbit/s, as specified in the standard. The base station BTS receives the data of the traffic channel and transfers it to the 64 kbit/s timeslot of the PCM circuit In addition, the three other traffic channels of the same carrier are inserted into the same timeslot, i.e. channel, resulting in that the transfer rate per connection is 16 kbit/s. At the BSC, the TRAU converts the coded 16 kbit/s digital information into the 64 kbit/s channel, and on this channel the data is transferred into the IWF unit at the MSC. The IWF carries out the necessary modulation and rate adaptation, after which the data is transmitted to some other network. Thus, user data is transferred over fixed connections in the uplink direction from the BTS to the BSC and the MSC, and correspondingly, the data to be transmitted to the MS is transferred in the downlink direction from the MSC via the BSC to the BTS and from thereon over the radio interface to the MS. The channel codec unit (CCU) of the base station carries out the conversion of the signal received on the radio channel into the channel of the PCM time slot in the trunk circuit over the A bis interface, and the conversion of the signal received over the A bis interface into the form transmitted to the radio path. The TRAU carries out the conversion operations for the signals to be transferred over the A interface.
The user data is transferred over the A bis interface from the BTS to the TRAU in a fixed-length TRAU frame. FIG. 2 shows in bit diagram form a TRAU data frame used to transfer a signal at the data rate of 13 kbit/s. The TRAU frame comprises 40 octets numbered 0, . . . , 39, its total length thus being 320 bits and duration 20 ms. Synchronization between the unit that transmits the TRAU frame and the one that receives it is achieved with synchronization bits that are shown in FIG. 2 as 0 bits and 1 bits. The 0 bits in the first two octets of the TRAU speech frame are used for carrying out the actual synchronization, and the 1 bits in the first bit position in the other even octets except the second and fourth, are used to ensure that elsewhere in the data frame there are no two-octet-long sequences of successive 0 bits that would look like a synchronization sequence. One TRAU speech frame contains 35 synchronization bits per the total number of 320 bits. The TRAU frame of FIG. 2 shows control bits C1-C21, timing bits T1-T4 and the user data bits denoted with X. Unused data bits are set to 1-state, for example, for the duration of breaks in the data transmission, whereby the TRAU frame to be transferred is in accordance with the idle speech frame of FIG. 3. The BFI control bit (Bad Frame Indicator) in the idle frame is used to indicate that the frame does not contain speech information.
The bits in the TRAU frames are normally transferred over the PCM line in a PCM frame according to ITU-T Recommendation G.704, whose structure is shown in FIG. 4. The frame comprises 32 octets numbered 0, . . . , 31. The frame duration with the transfer rate of 2 Mbit/s is 125 xcexcs. The 0-bits of the first octet are the frame synchronization. CH1, CH2 and X-bits marked in FIG. 4 comprise the bits to be transferred from different TRAU frames, so that at the transfer rate of 8 kbit/s the bit to be transferred from each TRAU frame i.e. channel is inserted into one bit position in the PCM frame, and at the transfer rate of 16 kbit/s user data bits of the TRAU frame are inserted into two bit positions in the PCM frame. FIG. 4 shows a PCM frame with the transfer rate of 16 kbit/s as regards the transfer of channels CH1 and CH2.
In the GSM system, the transmission of bit streams from different sources is enhanced e.g. by means of multiplexing and concentrator equipment. The bit steams are interleaved to e.g. 2 Mbit/s buses with TDM (Time Division Multiplex) so that each channel in the system is allocated a dedicated timeslot, which the channel always uses.
Discontinuous transmission (DTX) refers to a method in which the mobile station""s transmission to the radio path may be interrupted for the duration of pauses in speech. The aim is to reduce the power consumption of the mobile station, a very significant issue for it. In the GSM system, for example, the speech activity of the signal transmitted from the mobile station to the bas station is monitored at the mobile station, and if no speech information exists, the mobile station""s transmission to the radio path is cut. When the mobile station""s MS transmission has been discontinued, the base station BTS generates idle frames according to FIG. 3 and transmits them forward to the base station controller BSC. When speech again begins, it is coded at the mobile station and transmitted to the base station in the correct timeslot. In order for the receiving party not to sense the transmission cut as unpleasant total silence, comfort noise parameters (SID, Silence Descriptor) are transmitted to the base station at specific intervals, 0.5 s in the GSM system, which are used to generate comfort noise simulating background noise in the speech decoder.
The contents of the user bits in the TRAU frame are indicated with the aid of control bits. In the uplink direction, control bits C13 and C14 in the BFI, SID and TAF (Time Alignment Flag) flags in the TRAU frame are used to indicate whether the TRAU frame to be transferred contains speech or SID comfort noise parameters. In the idle TRAU frame according to FIG. 3, these control bits are employed to indicate that the frame does not contain payload. Correspondingly in the downlink direction, control bit C16 in the TRAU frame according to FIG. 2 is used to indicate whether the TRAU frame to be transferred contains speech (SP=1) or something else than speech information (SP=0).
The problem with the prior art speech transfer described above is that useless information is transferred in it, for example idle TRAU speech frames during pauses in speech, which leads to transfer capacity being wasted. Due to the fixed channel allocation of TDM (Time Division Multiplexing), the channel is continuously reserved for use by one traffic source regardless of the actual demand for transfer capacity of said traffic source. Because in speech traffic the subscribers as a general rule speak alternately, and because the speaking party will have irregular pauses when talking, the efficiency of the reserved channel is roughly less than 50% of the duration of the call. Particularly when connecting the traffic of several base stations to the same bus the average efficiency of the bus is a mere 30% of the theoretical maximum utilization degree because calls are often set up at different times, which means that only part of the channels on the bus are used simultaneously. With the prior art transfer, additional problems are caused by redundancy in the transfer of synchronization bits, caused by channel-specific synchronization of the TRAU frame structure when 35 synchronization bits are transferred in each TRAU frame.
The aim of this invention is to enhance data transfer so that more payload can be transferred with low transfer capacity than previously.
The object is achieved with the inventive method, characterized by that which is claimed in the independent claim 1. The preferred embodiments of the invention are disclosed in the dependent claims 2-11.
The invention additionally relates to a statistical multiplexer, a statistical demultiplexer and a transmission frame which are characterized by that which is claimed in the independent claims 12, and 16. The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on the idea that statistical multiplexing is employed to assign bus capacity only for the active channels. In this process, the bits of the active channels are placed into the transmission blocks of a variable-length transmission frame and sufficient identification data is added to the frame. The multiplexer sends this inventive transmission frame to the transmission bus whose capacity is advantageously in the exclusive use of the frame in question. On the transmission link, the frame is routed in the network nodes to the correct terminal node, in which the bits in the frame are demultiplexed to their respective channels on the basis of the identification data. In other words, the inventive statistical multiplexing transfers the bits from a prior art regular frame structure to the inventive variable-length transmission frame for the duration of the transfer, and back to the regular transmission frame at the receiving end. Additionally, information transfer is further enhanced in the particular embodiments of the invention, by minimizing the number of bits to be transferred by e.g. removing unnecessary synchronization bits and/or control bits, and possibly by compressing the assembled transmission frame prior to transfer.
Such statistical multiplexing provides the advantage that channel allocation is extremely dynamic, resulting in lower data transfer capacity and, secondly, the assigned capacity being brought into more efficient use. By assigning data transfer capacity for use by data transfer of the active channels, only, temporally-interleaved signals can be inserted in the same transmission block. With the inventive statistical multiplexing, the operator is able to add capacity to its network without further investments in transmission lines when e.g. more TRXs can be connected to one 2 Mbit/s transmission line than previously.